Summary:

Tetrabiblos ('four books'), also known in Greek as Apotelesmatiká, and in Latin as Quadripartitum, is a text on the philosophy and practice of astrology, written in the 2nd century AD by the Alexandrian scholar Claudius Ptolemy. Ptolemy's Almagest was an authoritative text on astronomy for more than a thousand years, and the Tetrabiblos, its companion volume, was equally influential in astrology, the study of the effects of astronomical cycles on earthly matters. But whilst the Almagest as an astronomical authority was superseded by acceptance of the heliocentric model of the solar system, the Tetrabiblos remains an important theoretical work for astrology. It is described as "indispensable" for serious students of astrology.